v1.2.0 released

Here is a summary of new features :
* 'subtime' class has been replaced by 'POSIXst' class.
* use 'POSIXst' methods to define instance of this class.
* SubtimeDataFrame now rely on 'POSIXst' objects.
* functions 'year', 'month', 'minute', 'second', etc. are now wrappers for the 'POSIXst' constructor.
* new methods have been added for 'POSIXctp' and 'POSIXcti' objects (tail, head, summary, split, match, %in%, unique)
* 'natural' mathematical operations between POSIXst (subtimes), POSIXctp (time periods) and POSIXcti (time intervals) are available.